In at least some arrangements, light emitting diodes (LEDs) have an active layer of semiconductor material sandwiched between n-type and p-type semiconductor doped layers. When a voltage is applied between the doped layers, an electric current is passed through the LED. Charge carriers, electrons from n-layer or holes from p-layer, are injected into the active layer where they recombine to generate light. The light generated by the active region emits in all directions and escapes the LED through all exposed surfaces (light emitting surfaces). The efficiency of LEDs is limited by the phenomenon of total internal reflection (TIR) in which a part of the light is reflected from the light emitting surface back into the LED and is lost due to light absorption. The greater the difference in refractive indices (n) of the materials at the light emitting surface compared to the environment to which the light exits (n=1.0 for air and n≈1.5 for epoxy), the stronger the negative impact of TIR. Typical semiconductor materials have a relatively high index of refraction (n≈2.2-3.8); therefore, much of the light generated by the active layer of the LED is blocked by the light emitting surface.
Green, blue, and ultraviolet LEDs can be manufactured, for example, with gallium nitride (GaN) epitaxially grown on substrates of sapphire (Al2O3), silicon carbide (SiC), silicon (Si), SiC-on-insulator (SiCOI), Si-on-insulator (SOI), or the like. Infrared, red, and yellow LEDs can be manufactured, for example, with ternary or quaternary compounds of A3B5 (Al,Ga,In)(P,As) grown on substrates of gallium arsenide (GaAs) or indium phosphide (InP). These compounds can include, in particular, aluminum containing semiconductor compounds from a group including AlAs, AlGaAs, AlGaInP, AlGaN, and AlGaInN.
The growth substrate is sometimes removed to improve the optical characteristics and to reduce the resistance of LED layers. A sapphire substrate, for example, can be removed by laser melting of GaN at the GaN/sapphire interface, and silicon and gallium arsenide substrates can be removed, for example, by selective wet etching.
One method for reducing TIR loss includes depositing on a growth substrate an n-type layer, an active layer, and a p-doped layer, forming a conductive substrate above the p-doped layer, removing the growth substrate to expose the n-doped layer, followed by photo-electrochemical (PEC) oxidation and etching of the n-doped layer to form a roughened surface to enhance the light extraction. A 2-fold increase in LED light extraction has been achieved by this method compared to an LED with a flat, light emitting surface. One disadvantage of this method is that a random distribution of roughness amplitude, up to 0.5 μm, can lead to a nonuniform distribution of current over the surface due to thickness nonuniformity in the n-type layer, which is often critical for thin-film LEDs with the n-type layer thinner than 2-3 μm.
One method for manufacturing thin-film LEDs includes growing the first and second epitaxial layers of different conductivity types with an active layer between them on a growth substrate, providing a package substrate with contact pads for the first and second epitaxial layers of individual LEDs, bonding the second epitaxial layer to the contact pads of the package substrate using a metal interface, removing the growth substrate, etching the exposed surface of the first epitaxial layer such that the LED layers have a thickness less than 10 μm or less than 3 μm, forming light extraction features in the primary emission surface to enhance the light extraction from an exposed light emitting surface of the first epitaxial layer which includes of roughening, patterning, and dimpling the primary emission surface, or forming a photonic crystal. The efficiency of a thin-film LED was enhanced both by surface features and by thinning the layers, removing the substrate absorbing the part of the light, making the reflecting contact at the side of the mounting substrate, and lowering the LED heating due to heat removal into the mounting substrate. However, the creation of micron- and submicron-sized roughness with a random profile is not consistent with the trend of thinning LED layers down to a total LED thickness of less than 3 μm.